Generally speaking, a plurality of traces consisted of copper materials is formed on a surface of a semiconductor package substrate and can be extended to form conductive pads which serve for signal transmission. In order to successfully electrically connect conductive elements (such as gold wires, solder bumps or solder balls) to a surface of a chip or circuit board, a metal layer such as nickel/gold (Ni/Au), nickel/silver (Ni/Ag) and the like needs to be plated on an exposed surface of the conductive pad and serves as an attachment layer between the conductive element and the conductive pad. In general, conductive pads known in the prior-art comprises a bump pad and a presolder pad for electrically connecting a flip-chip package substrate to a chip; a finger for electrically connecting a wire bonding package substrate to a chip; or a ball pad for electrically connecting a package substrate to other circuit boards. Thus, the body of the conductive pad can be prevented from being oxidized and solder joint reliability between the conductive element and the conductive pad can be improved using the nickel/gold metal layer formed on the surface of the conductive pad.
In general, prior-art methods for fabricating the nickel/gold metal layer on the conductive pad mainly comprise chemical nickel/gold fabrication, ion sputtering, plasma deposition and nickel/gold electroplating.
However, the prior-art chemical nickel/gold fabrication is inherent with significant reliability problems such as imperfect solderability and an insufficient intensity of a soldering point, for example, incomplete nickel/gold plated and a coarse nickel/gold surface. Referring to the cause of the incomplete nickel/gold plated problem, when a chemical tank is reheated after being cooled down for a while, depositing ability is insufficient to provide the full nickel/gold plated although all operating conditions are well prepared, such that gold cannot be successfully plated and copper is thus exposed. Further, referring to the cause of the coarse nickle/gold surface problem, when the surface of nickel is immersed with gold, nickel deposited underneath is continuously oxidized and aged by an enhanced effect of a chemical potential because of over oxidization of the surface of nickel, irregular deposition of large-sized gold atoms and porosity of rough gold crystals, such that nickel rust which is not melted away is continuously accumulated beneath the surface of gold. Thus result in a coarse nickel/gold surface. The foregoing incomplete nickel/gold plated and coarse nickel/gold surface problems caused by the chemical nickel/gold fabrication might easily result in detaching the gold wire, solder bump, presolder and solder ball from the conductive pad, so as not to provide a good electrical connection. Additionally, the costs of ion sputtering and plasma deposition are too high to comply with the economic efficiency.
Therefore, an electroplating process is commonly employed to form the nickel/gold metal layer on the conductive pad. Referring to FIG. 1, the prior-art fabrication of the nickel/gold metal layer using an electroplating process relates to define a plurality of conductive pads 14 (such as fingers and ball pads) on a substrate 1 by developing and etching techniques, wherein the substrate 1 has been previously subject to former fabrication including circuit patterning of upper and lower circuit layers 11 and 12 and formation of a plurality of plated through holes 13 penetrating through the substrate 1. Further, a solder mask layer 15 is also formed on an outer surface of the substrate 1.
Referring to FIG. 1, the structure of a nickel/gold metal layer 16 is formed on the conductive pad 14 by an electroplating process. However, in order to form such structure, a plurality of plating traces 17 connected to original traces has to be additionally provided on the substrate 1 to electroplate the nickel/gold metal layer 16 on the conductive pad 14. Thus, although the nickel/gold metal layer 16 can be formed on the conductive pad 14, a plurality of plating traces also needs to be provided to perform the electroplating process. Consequently, not only areas of the substrate 1 free of arranging circuits are occupied but also an antenna effect might be caused by extra plating traces to therefore result in noises during the high frequency use. If an etchback method is employed to remove the plating traces 17, an end portion of the plating trace will still be remained. Thus, although the nickel/gold metal layer can be formed on the conductive pad 14, an irregular structure caused by the end portion of the plating trace will be existed. Problems such as limited areas of the substrate for circuit arrangement and noise interruption during the high frequency use are existed. Additionally, as the nickel/gold metal layer is also formed on a surface of the plating trace 17 while forming the nickel/gold metal layer 16 on the conductive pad 14, and a number of etchback processes have to be subsequently performed to remove the plating trace 17 which has no functions, the substrate is seriously scraped and damaged. Moreover, referring to the conditions of complexity of circuit arrangement of the substrate and high density conductive pads, the remained area of the substrate is insufficient for arranging additional plating traces, therefore increasing the difficulty in fabricating the nickel/gold layer using an electroplating process.
Another electroplating process widely employed by manufacturers is “gold pattern plating (GPP)”. Referring to FIG. 2A to FIG. 2D, first of all, a conductive layer 21 (as shown in FIG. 2A) is respectively formed on upper and lower surfaces of a substrate 2. Then, a plurality of plated through holes (PTH) or blind vias (not shown) are formed penetrating through the substrate 2 to electrically connect the conductive layers 21 formed on the upper and lower surfaces of the substrate 2.
Referring to FIG. 2B, a photoresist layer 22 is formed on the conductive layer 21 of the substrate 2. The photoresist layer 22 is provided with a plurality of openings to expose the conductive layer 21 for subsequently forming circuit areas. The conductive layer 21 serves as a current conductive path, such that a nickel/gold metal layer 23 is formed on a surface of the conductive layer 21 free of being covered by the photoresist layer 22 using an electroplating process.
Referring to FIG. 2C, the photoresist layer 22 is removed. Then, the nickel/gold metal layer 23 is used as a mask resist layer, and the conductive layer 21 underneath the nickel/gold metal layer 23 is patterned using an etching technique. Therefore, referring to FIG. 2D, the nickel/gold metal layer 23 is covered on a surface of a circuit pattern 21a formed by etching the conductive layer 21.
Instead of the plated traces, the foregoing gold pattern plating (GPP) technique employs the conductive layer to serve as the current conductive path for electroplating the nickel/gold metal layer. However, as the whole circuit layer of the substrate (comprising the conductive pad and all traces) is formed with the nickel/gold metal layer on a surface thereof, the material cost is extremely high. Also, during a subsequent circuit pattering process, the solder mask layer and the nickel/gold metal layer cannot easily be adhesive to each other due to a difference between these two materials, so as not to achieve a stable structure.
Therefore, the problem to be solved here is to provide a method for fabricating a semiconductor package substrate having a plated metal layer on a conductive pad, by which fabrication procedures can be simplified and a fabrication cost can be minimized to solve problems such as incomplete nickel/gold plated and a course nickel/gold surface caused by the prior-art chemical nickel/gold fabrication while eliminating drawbacks such as imperfect reliability and wasting of costs caused by the prior-art fabrication of the nickel/gold metal layer using an electroplating process.